Production of adiponitrile

ABSTRACT

Process for the production of adiponitrile by direct electrochemical hydrodimerization of acrylonitrile using a medium containing acrylonitrile, an electrolyte salt, water and, if desired, a solvent at a temperature of from 10* to 60* C. at a pH of from 1 to 10. In the process, electrolysis is carried out in a cell having a liquid cathode which is contacted by a solid anode which is not wetted by the liquid cathode or into which the solid anode is immersed to a depth of up to 20 mm. Adiponitrile is an important intermediate for synthetic fiber manufacture.

United States Patent Beck et al.

I 541 PRODUCTION OF ADIPONITRILE [72] Inventors: Fritz Beck, Ludwigshafen; Harald Guthke, Frankenthal; Hans Leitner, Ludwigshafen, all of Germany Badische Anilin- & Soda-Fabrik Aktien- [73] Assignee:

g ll i afa udw shsfw n1, Rhine-Ptalz, O e rrnany MW [22] Filed: Mar. 17, 1969 21 Appl. No.: 807,895

[30] Foreign Application Priority Data Mar. 16, 1968 Germany ..P 16 93 005.5

[52 vs. 01 ..204/73, 204/250 51 Int. Cl. ..C07b 29/06 [58] Field of Search ..204/72, 73, 250

[56] References Cited UNITED sTATEs PATENTS 3,193,574 7/1965 Katchalsky et al ..204/73 3,193,479 7/1965 Baizer ..204/72 Feb. 15, 1972 3,193,477 7/1965 Baizer ..204/72 FOREIGN PATENTS OR APPLICATIONS 1,146,481 4/ 1963 Germany ..204/99 1,404,896 5/1965 France ..204/99 Primary Examiner-John l-I. Mack Assistant Examiner-R. L. Andrews Attorney-Johnston, Root, OKeeffe, Keil, Thompson & Shurtleff [57] ABSTRACT Process for the production of adiponitrile by direct electrochemical hydrodimerization of acrylonitrile using a medium containing acrylonitrile, an electrolyte salt, water and, if desired, a solvent at a temperature of from 10 to 60 C. at a pH of from 1 to 10. In the process, electrolysis is carried out in a cell having a liquid cathode which is contacted by a solid anode which is not wetted by the liquid cathode or into which the solid anode is immersed to a depth of up to 20 mm. Adiponitrile is an important intermediate for synthetic fiber manufacture. I

8 Claims, 2 Drawing Figures PATENTEBrmsmz 3.642.592

SHEET 1 BF 2 INVENTORS. FRITZ BECK HANS LE/TNEI? PRODUCTION OF ADIPONITRILE This invention relates to the production of adiponitrile by electrochemical hydrodimerization of acrylonitrile.

The possibility of preparing adiponitrile by electrochemical hydrodimerization of acrylonitrile according to the empirical equation:

2CH CHCN+2H O+2 NC(CH ),CN+2OH is of great practical interest because a high yield of the desired substance is obtained from an easily accessible starting material and the purity of the product is outstanding. In recent years quite a number of methods for carrying out this reaction have become known, for example direct electrolysis in cells divided by diaphragms or in undivided cells or indirect electrolysis by reaction of acrylonitrile with alkali metal amalgam and electrolytic production of the alkali metal amalgam. More recently catalytic hydrodimerization over ruthenium catalysts has been described, however the yields of adiponitrile are low owing to the formation of large amounts of propionitrile.

All prior art methods have at least two of the disadvantages enumerated below:

i. complicated cell design;

ii. unfavorable cell voltages at industrially adequate current densities;

iii. protracted processing;

iv. low yields; and

v. formation of inorganic byproducts.

It is an object of the present invention to provide a process for the electrochemical hydrodimerization of acrylonitrile which gives high yields of adiponitrile in a simple electrolytic cell. Another object is to provide a process which does not give inorganic byproducts and in which the product is easy to process. Another object is to provide a process in which favorable cell voltages are possible at industrially adequate current densities.

In accordance with this invention, these and other objects and advantages of the invention are achieved in a process for the production of adiponitrile by direct electrochemical hydrodimerization of acrylontrile using a medium containing acrylontrile, an electrolyte salt, water and, if desired, a solvent at a temperature of from to 60 at a pH of from 1 to l0 in which electrolysis is carried out in a cell having a liquid cathode which is contacted by a solid anode which is not wetted by the liquid cathode or into which the solid anode is immersed to a depth of up to mm.

The starting material, auxiliaries and reaction conditions to not differ from those used in prior methods.

An electrolysis mixture which contains 30 to vantageously from 50 to 65 is generally used.

Water is used as a proton donor, usually in a concentration of from 3 to 40 percent; larger or smaller water contents, for example down to 1 percent or up to 94 percent, may however be used. It is advantageous to use mixtures which form a homogeneous liquid phase. It is possible to use both solutions of acrylonitrile in water and solutions of water in acrylonitrile. Although the reaction may be carried out without a solvent or diluent, it may be advantageous to use polar solvents in order to set up specific concentrations of acrylonitrile or of water in the mixture. Such solvents may either be inert or may to some extent undergo change during the reaction and may also themselves act as proton donors. The following are particularly suitable as solvents: lower aliphatic alcohols such as methanol, ethanol and isopropanol; acetonitrile; ethers which are partly or wholly miscible with water such as tetrahydrofuran, dioxane and glycol monomethyl ether; or open or cyclic amides of lower carboxylic acids which may be substituted by alkyl or dialkyl on the nitrogen atom, for example forma'mide, monomethylformamide, dimethylformamide, dimethylacetamide, diethylacetamide and N-methylpyrrolidone.

It is advantageous to add to the electrolysis mixture a small amount of a substance which is more easily oxidized anodically than the electrolyte salt, acrylonitrile or adiponitrile. Anodic oxidation of the starting material or reaction product which occurs as a secondary reaction and results in loss of 90 percent, adpercent, by weight of acrylonitrile yield is suppressed by such an addition. Examples of suitable substances are lower alcohols, particularly methanol, and lower aldehydes. The substance which is more easily oxidized is advantageously added to the reaction mixture in an amount of from 5 to 30 percent by weight. If the whole of the oxygen normally formed in the reaction was to be used up for the oxidation of methanol, about g. of methanol per kg. of adiponitrile would be required.

It is preferred to use quaternary ammonium salts of certain acids, preferably in concentrations of less than 5 percent by weight, as electrolyte salts. The salt may however be used in higher concentrations, for example up to 20 percent by weight. The cations of the salts have a very negative discharge potential. Those anions are particularly suitable which are only difficulty oxidizable or not at all, for example monoalkyl sulfates, sulfate, fluorides, tetrafluoroborates, fluorosulfonates toluenesulfonates and benzene sulfonates. It is preferred to use tetramethylammonium and tetraethylammonium salts, particularly of ethyl sulfate and p-toluene sulfonate. Other salts which have proved to be suitable are for example triethylmethylammonium methyl sulfate, bis-tetraethylammonium sulfate, bis-tetrabutylammonium sulfate, tetramethylammonium methyl sulfate and tetramethylammonium fluorosulfate. Mixtures of salts may also be used, sometimes with advantage. Mercury is particularly suitable as cathode material because of its highway hydrogen and because it is liquid. Gallium is also suitable at temperatures above 30 C.

The anode should not dissolve in the reaction mixture, particularly in the case of anodic polarization, and the anode material should not be'wettable by the liquid cathode. These requirements are generally fulfilled by graphite or oxide anodes. Electrographites of all types are therefore suitable preferably compacted grades which have less of a tendency to disperse in the electrolytes, and also lead dioxide (either as solid shapes or better as a coating on electrode carbon), nickel or. titanium, and also magnetite electrodes which have been prepared by sintering or centrifugal casting methods. Metallic e.g., platinum or gold anodes are suitable provided an anodic covering layer has been produced by appropriate pretreatment, for example by prolonged anodic prepolarization. Alloys of tungsten with from 10 to 20 percent of nickel are also favorable, especially after anodic prepolarization in dilute acids, preferably phosphonic acid. The shape of the anode is preferably cylindrical with rounded edges on the front face to be immersed. Other shapes, for example hemispheres, truncated cones or concave logarithmic or spherical caps may also be used. The anodes may also have a rectangular or square cross section or be segments of a circle.

Reference will now be made to the drawings.

in FIG. 1 a prototype of an electrolytic cell for carrying out the process according to this invention is shown diagrammatically. The cell casing 1 (made from polyolefin or from steel coated with polyolefin for example) has the flat and elongated shape conventionally employed in chlorine/caustic cells using the amalgam method. Its width depends on the diameter of the anode 2 and its length on the number of anodes. The anodes 2 have a cylindrical shape; the edges of the lower face are rounded. A bus 3 is connected to a source of electricity. Circular depressions 4 are provided in the bottom of the cell and these have a somewhat larger diameter than the anodes and are partly filled with the liquid cathodes. Current is supplied to the cathodes through an iron ring 6 and a busbar 7. The anodes 2 dip into the liquid cathode to a depth of from 0 to 10 mm. The reaction mixture 8 is pumped from below through nozzle 9 so that a thin coherent film of liquid is ensured between the anodes and cathodes. The reaction mixture leaves the cell through an overflow l0 and is returned to the cell by a pump 1] through a heat exchanger 12 and the nozzles 9. Temperature and pH are measured by a thermometer l3 and electrodes 14; the pH is controlled by adding a suitable base through line 15. Offgas formed during the electrolysis escapes through a cooled offgas pipe 17 which passes through a gastight cover 16.

The individual cathode/anode units in a cell may be connected in parallel according to FIG. 1. Owing to the low electrolytic conductivity of the electrolysis mixture (which reaches only about 1 percent of the conductivity in chlorinecaustic cells) it is also possible however to connect the pairs of electrodes in a trough in series without corrosion occurring at the electrodes. The provision of vertical partitions between the pairs of electrodes and having a height not extending completely to the surface of the electrolysis mixture is advantageous in this case.

It is furthermore advantageous for the anodes to be prepolarized at a potential of about volts for from 10 to 30 minutes while withdrawn, i.e., at a distance ofa few mm from the liquid cathode, prior to electrolysis. Stabilization of the system during electrolysis is thus achieved.

During electrolysis the anode has a depth of immersion of from 0 to mm. in the liquid cathode, i.e., it either rests on it or dips into it a little. The current strength at constant potential increases with increasing depth of immersion, but only at the rate at which the contact area between anode and cathode increases.

Other arrangements may be used instead of the central nozzle, for example in the case of larger anode faces, two rakeshaped nozzles which cross over each other, in the case of pyramidal anode faces, appropriately crossed comb-shaped nozzles, and in the case of rectangular anodes a slot nozzle extending over the whole width at the lower end of the anode.

It is also possible to impart rotary motion to the anodes. Cylindrical anodes may for example be rotated about their vertical axes. A particularly favorable arrangement is a rotating circular disc or roller having a horizontal axis which may dip in the cathode to a depth of up to half its diameter. The portion which is not immersed is constantly wetted by fresh solution so that it is not necessary in this case to bathe the anodes.

After the anode has been contacted or dipped, electrolysis can be carried out without further measures. Exchange ofsubstance between the zone of contact and the surrounding reaction medium evidently is ensured by capillary forces. It is however advantageous in order to withdraw heat and ensure steady-state operation to pump the electrolyte upwardly through the liquid cathode against the anode. lt has proved to be particularly advantageous to use for this purpose a perforated ring nozzle (see reference 9 in FIGS. 1 and 2) which is located coaxially with the anode beneath the center of the face of the same. The nozzle may for example be secured to the anode so that uniform distribution is achieved; the feed line obviously has to be elastic in this case. The electrolyte may however also be passed through a coaxial bore in the anode to the lower face of the anode. By bathing the zone of contact with fresh electrolyte, an improvement in the currentpotential relationship is achieved as may be seen from Table l. The experimental conditions are the same as in Example 1 with the exception of the anode material. The test is carried out with a cylindrical anode having a cross-sectional area of 1 cm.

Very favorable current/potential ratios are achieved in electrolysis by the process according to this invention even in the case of low concentrations of salt, as may be seen from Table 2. The test conditions are the same as in Example 1, i.e., the anode consists of graphite coated with lead dioxide.

TEAES =Z by weight oftctraethylammonium ethyl sulfate.

From these results it will be seen that the electrolysis can be carried out with the arrangement according to this invention, even in the case of low concentrations of salt, at moderate potential with current densities of from 20 to amp./dm. i.e., in the range of current densities conventionally used in the electrolysis of alkali metal chlorides.

As regards the pH during electrolysis, it has been found that surprisingly it is possible with cells according to this invention to use unusually low pH values without more byproducts (propionitrile) being formed. Whereas in the prior art methods the favorable pH range is from 7 to 10, it is advantageous in the process according to this invention to maintain a pH offrom 3 to 6, although a higher pH may be used, for example up to 10. The pH may be lowered down to l. The lower pH value has a very favorable effect on the consumption of quaternary ammonium hydroxide for regulating the pH. The specific consumption of base for maintaining a constant pH of from 7 to 8 is from 0.5 to 1.0 millimole per ampere hour, while at a pH of from 3 to 4 it is only 0.05 to 0.1 millimole per ampere hour.

The electrolysis is carried out at a temperature of from l0 to 60 C., preferably from 25 to 40 C.

Conversion of the acrylonitrile is from 10 to 70 percent. Electrolysis may be carried out continuously by allowing the reaction mixture to circulate, for example according to FIG. 1 or 2, and after the desired conversion has been achieved keeping it constant by metering in freshreaction mixture while at the same time withdrawing reacted mixture from the cell.

Owing to the low concentration of salt, it is very simple to process the discharge from the electrolysis. The components of low boiling point, namely unreacted acrylonitrile, solvent, propionitrile and water and the components of high boiling point as well as the cyanoethylation products of water and possibly of the alcohol may be isolated direct by distillation. Oligomeric acrylonitrile and salt remain as residue.

The fact that the graphite anode in alkali-caustic cells can be immersed in the mercury cathode without shortcircuiting occurring has been known for years (see for example German printed application No. 1,146,481 and French Pat. specification No. 1,404,896). The process according to this invention differs from the prior art methods however in the following important points:

a. an organic compound is reacted; it is only the electrolysis of inorganic compounds in cells with contacting electrodes which has been known;

b. there is no critical current density which has to be exceeded in order to prevent shortcircuiting. In the said printed application and patent specification it is disclosed that a current density of 70 to 200 amp./dm. has to be exceeded at a depth of immersion of 0.2 to 5 mm. in order to prevent shortcircuiting. In the process according to this invention the electrolysis can be carried out under completely steady-state conditions at 5 amp./dm. (3.8 v-);

c. The reason given in the said Printed Application and Patent Specification why shortcircuiting does not occur is that a cushion of chlorine gas forms beneath the anode and makes shortcircuiting impossible. This explanation cannot apply to the process according to this invention because evolution of gas (carbon dioxide) at the anode is relatively slight; at lower current densitie u h a 5 to 20 sure. An analysis for sulfur and nitrogen is carried out in the ampjdm. it is scarcely visible. residue and the proportion of oligomeric acrylonitrile is calcu- In the following examples: lated from it. Propionitrile in the low boiling fraction, and PN= propionitrile adiponitrile, bis-cyanodiethyl ether, B-hydroxypropionitrile. AN= adiponitrile 5 succionitrile and other byproducts in the high boiling fraction CEE=bis-cyanodiethy1 ether are determined by gas chromatography. The yields are given HPN= fl-hydroxypropionitrile in Table 3 together with two other experiments with conver- SN= succionitrile sions of 20 and 30 percent: R= residue Con= Conversion percent BP= other byproducts l Con s= Consumption of TMAH in millimole per ampere TEAES=tetraethy1ammonium ethyl sulfate .Jlour TEAMS= tetraethylammonium methyl sulfate I h TMAH= tetrarnethylamm'onium hydroxide g TABLE 3 Yields given in the tables are with reference to acrylonitrile Yield (percent) Current efli l reamed, v jcy (percen GEE l The invention illustrated by the following examples. Con AN p HPN SN r p R AN pN Cons EXAMPLEI 85.7 1.0 0.7 1.4 11.4 in 2 0.1? Electrolysis is carried OUIYII'I an arrangement as shown dia- & 0, ,0 1 ,3 85 3 0, grammatically in FlG.--2,in which the references corresponds 33 5 12 to those in FIG. 1. A glass cell 1 contains a cylindrical anode 2 0 1 Plus. of graphite covered with a layer of lead dioxide having a thickness of 50 microns. Current is passed in through a copper lead 3. The diameter of the anode is 44 mm. and it has a front y EXAMPLE 2 face having anarea of 15 cm.. During electrolysis it is im- 2 Electrolysis is carried out in the apparatusdescribed in Exmersedto a depth of 2 mm. into mercury 5 covering the botample. 1 under the conditions specified therein with the tom 4 of the cell. Supply of current-to this mercury cathode is modification that F10 amperes (i=67 amp./dm. Electrolyeffected through a platinum pin 6 which is fused into a glass sis is discontinued after a theoretical current conversion of 20 tube and welded to a copper lead 7. The reaction mixture is percent. pumped upwardly through a perforated ring nozzle 9 made of 30 Yields obtained at various pH values, together with the polyethylene and leaves the cell by an overflow 10. The reacmean cell potential and the specific consumption of base are tion medium is recycled through a cooler 12 by a pump 11. compared in Table 4. v

TABLE 4 Yield (percent) Current efflclency (percent) CEEl V0 Cons pH" AN PN HPN SN 131 R AN PN (Volts) 'IMAH a3. 1.0 0.5 as 10.3 32 2 7.0 0.02 i 84.5 1.7- 0.6 2.2 11.1 85 3 7.8 0.15 5.. 84.3 0.9 0.4 0.4 11 12.0 33 2 8.1 0.35 6 "84.5 1.9 0.8 0.4 16 11.0 75 3 8.0 0.40-

1 Plus.

Tne temperature is measured by means of a thermometer i3 7 m EXAMPLEB v and the pH is measured by means of an electrode 14. The pH Electrol sis is carried out in the a aratus described in Exkeplt 2 -1 g 2M a dmppmg ample l uiider the conditions of Ex iiiple 2 (r-'67amp./dm. unne ending g z z f fi gf t mug a nnevcooler 17 ex pH 4,20 percent conversion) but with the followtngconcenf salt: At the beginning of the electrolysis 1,000 g. ofa mixture of 54 percent by weight of acrylonitrile, 28 percent by weight of 1% TEAES isopropanoll t isopropanol, 16 percent by weight of water and 2 percent by 2% TEAES 54 0 0 lsopropano wa weight of TEAES is placed inthe apparatus. The anode i f t .A.N 2 l BP PQDQ prepolarized at V=l0 volts-and i=0.3 amp. for about 15 Yields and mean cell potentials in these experiments are minutes without contacting the mercury.-Then the anode is collecwdin Table 55 slowly dipped into the mercury cathode to a depth of 2 mm 55 E' X= Experiment.

TABLE 5 Yield (percent) Current eflicleney GEE (percent) plus 0 Experiment AN PN HPN SN BP R AN PN (volts) The current strength is adjusted to 15 amp., i.e., a current den- EXAMPLE 4 sity of 100 amp./dm. The mean cell potential Vc during the electrolysis is 10.4 volts. The temperature is kept at 38 C. After 7.28 hours, 109.1 ampere hours and a theoretical conversion-of the acrylonitrile of 40 percent, the electrolysis is discontinued. The reaction mixture is clear and almost color Electrolysis is carried out in the apparatus described in Example 1 under the following conditions, i. e., using other cosol vents:

a.'20% methanol, 2% TEAES 67% acrylonitrile, l 1% water less. The specific consumption of base is 0.12 millimole per j=67 amp./dm. V=8.l volts, 25% conversion; 30 C; pH ampere hour'. 4.5

The whole is worked up by distillation of the low boiling b. 30% dimethylformamide, 1.5% TEAES, 48.5% point constituents in a film evaporator followed by distillation acrylonitrile, 20% water j=32 amp./dm. V=8.l volts,

of the high boiling point constituents at subatmospheric pres 25% conversion, 23 C, pH 3.4.

Yields and mean cell potentials in these experiments are given in Table 6:

TABLE 6 Electrolysis is carried out under the conditions of Example 2 (pl-i=4, 20% conversion) but with i=30 amp corresponding to j=3 7.5 amp./dm. in the apparatus described in Example I but with an anode having a diameter of 100 mm. and a front face area of 80 cm The composition of the electrolyte is 54% of acrylonitrile, 28% of isopropanol, 16% of water, 1.5% of TEAES and 0.5% of TEAMS. The specific base consumption is 0.06 millimole per ampere hour. Working up gives yields with reference to acrylonitrile reacted of: AN=84.3%; PN=3.0%; CEE+HPN=0.7%; SN=%; BP=4.l% and R=7.9%. Current efficiencies: AN 89%; PN 6%. The cell potential at the beginning of the test is 6.9 v. and at the end 6.5

EXAMPLE 6 Electrolysis is carried out under the conditions specified in Example 2 (i=67 amp./dm. 20% conversion) but at pH=5 and 1 percent TEAES in the apparatus described in Example 1 but with an anode of graphite without a layer of lead dioxide. The cell potential is volts. The specific base consumption is 0.30 millimole per ampere hour. Yields with reference to acrylonitrile reacted are: AN=76.6%, PN=4.0%, CEE+HPN= 0.5%, SN=0.5%, B%2.4%, R=l6%. Current efficiencies are: AN=69%, PN=7%.

EXAMPLE 7 Electrolysis is carried out under the conditions given in Example 2 (i=67 amp./dm. 20% conversion) but at pH=S in the apparatus described in Example 1 but with an anode which rotates slowly about its vertical axis at the rate of lOO r.p.m. The electrolyte is pumped through a conventional glass nozzle situated 5 mm. beneath the center of the lower face of the anode. The cell potential is 8.4 volts. Yields with reference to acrylonitrile reacted are: AN=87.0%, PN=2.2%, CEE+HPN= 0.6%, SN=O%, BP--2.0%, R=8.2%. Current efficiencies are: AN=8 1%, PN=4%.

We claim:

1. A process for the production of adiponitrile by direct electrochemical hydrodimerization of acrylonitrile using a medium containing acrylonitrile, an electrolyte salt, and water at a temperature of from 10 to 60 C. at a pH of from 1 to 10 wherein direct current is passed between a liquid cathode and a solid anode which is not wetted by the liquid cathode said anode resting upon or being immersed in the liquid cathode to a depth ofup to 20 mm.

2. A process as in claim 1 wherein an anode having a surface of lead dioxide is used.

3. A process as in claim 1 wherein the surface of the anode which contacts the cathode is continually bathed with the electrolyte mixture.

4. A process as in claim 3 wherein the electrolysis mixture is allowed to flow upwardly through the liquid cathode to the anode.

5. A process as in claim 1 wherein a medium is used which contains from 5 to 30 percent by weight of a lower aliphatic alcohol.

6. A process as in claim 5 wherein the lower aliphatic alcohol is isopropanol.

7. A process as in claim I wherein a polar solvent for acrylonitrile is present in said medium.

8. A process as in claim 1 wherein said anode is a graphite or oxide anode and wherein said liquid cathode is mercury. 

2. A process as in claim 1 wherein an anode having a surface of lead dioxide is used.
 3. A process as in claim 1 wherein the surface of the anode which contacts the cathode is continually bathed with the electrolyte mixture.
 4. A process as in claim 3 wherein the electrolysis mixture is allowed to flow upwardly through the liquid cathode to the anode.
 5. A process as in claim 1 wherein a medium is used which contains from 5 to 30 percent by weight of a lower aliphatic alcohol.
 6. A process as in claim 5 wherein the lower aliphatic alcohol is isopropanol.
 7. A process as in claim 1 wherein a polar solvent for acrylonitrile is present in said medium.
 8. A process as in claim 1 wherein said anode is a graphite or oxide anode and wherein said liquid cathode is mercury. 